fozgometer readings all of a sudden much higher after retip


My cartridge used to read right around the center 20.   Now I am reading around 30.  Just had the cartridge retipped by Andy.  Seems a bit weird.  What could be causing this?
tzh21y
The Foz is very sensitive to the battery that's powering it.  If I haven't used my Foz for a couple of months, I get a new battery for it.
Thanks for the feedback.  Just seemed like they always read right around 20(center).  I will try a new battery as the one inside is older